Heat treating apparatus using quartz glass

ABSTRACT

A quartz glass which would not become a source for the contamination even if it contains metallic impurities. This quartz glass includes a region where a concentration of E′ center as measured by means of an. electron spin resonance analysis is 3×10 19  cm −3  or more. This quartz glass can be manufactured by a method including the steps of forming an initial quartz glass by melting and quenching a raw material for quartz glass, and implanting therein an ion, which is capable of entering into an SiO 2  network of the initial quartz glass and substantially incapable of externally diffusing, to increase a concentration of E′ center in at least part of the initial quartz glass. This quartz glass can be manufactured by a method making use of a quartz glass raw material containing 0.01 to 0.1% by weight of-silicon, by a method of irradiating ultraviolet ray to the initial quartz glass, or by a method of giving an abrasion damage to the surface of the initial quartz glass by means of sand blast.

This is a division of application Ser. No. 09/584,721, filed Jun. 1,2000; now U.S. Pat. No. 6,263,704, and which is a division of parentapplication Ser. No. 09/085,006, filed May 28, 1998, now U.S. Pat. No.6,093,666.

BACKGROUND OF THE INVENTION

This invention relates to a quartz glass, in particular to a quartzglass for use as a structural material for a heat treating apparatus tobe employed in a semiconductor manufacturing process, and to a methodmanufacturing the quartz glass. This invention also relates to a heattreating apparatus using the quartz and to a heat treating method usingthe quartz.

A heat treating apparatus is employed as one of the apparatus to beemployed in a semiconductor manufacturing process. In the heat treatingapparatus of this kind, a quartz glass is employed as a structuralmaterial for a furnace tube, a wafer boat, etc., because the employmentof quartz glass is advantageous in various aspects, e.g. it is availablein high purity or it is excellent in heat resistance.

Even if a quartz glass of a high purity is employed as a structuralmaterial, it is difficult to prevent the surface of a silicon wafer frombeing contaminated with a metal such as Cu and Fe when the silicon waferis heat-treated at a high temperature of 1,000° C. or more. The percentdefective of a semi-conductor device which is manufactured by making useof a silicon wafer containing a relatively large quantity of metalliccontaminants originated from a heat treating furnace is higher than thatof a semiconductor device manufactured by making use of a silicon waferwhich is free from the metallic contaminants.

The causes for the contamination of a silicon wafer with metals in itstreatment inside a heat treating furnace may be attributed to thefollowing facts.

First of all, metallic impurities which are contained in variousexternal members (such as a heater, liner tube made of silicon carbide,etc.) disposed outside the quartz glass furnace tube and are large indiffusion coefficient in quartz glass are caused to evaporate from thesurfaces of these external members, the evaporated metallic impuritiesbeing subsequently adsorbed on the external wall of the quartz glassfurnace tube. The metallic impurities thus adsorbed are then diffusedinto the interior of the quartz glass furnace tube to reach the innerwall of the furnace tube, from which the metallic impurities are causedto be desorbed whereby contaminating a silicon wafer with the desorbedmetallic impurities (a first cause).

Secondary, metallic impurities originally contained in the quartz glassfurnace tube are caused to diffuse up to the surface of the furnacetube, from which the metallic impurities are caused to be desorbedwhereby contaminating a silicon wafer with the desorbed metallicimpurities (a second cause).

For the purpose of minimizing the metallic contamination to be broughtabout by these causes, a method of improving the purity of bulk of theconstituent members of heat treating apparatus has been mainly adopted.For example, in the case of the quartz glass furnace tube, the followingmethods have been adopted to improve the purity of the quartz glass.

Namely, a method of enhancing the purity of natural quartz crystal as araw material for quartz glass (a first method); a method of employingsilicon tetrachloride or a synthetic amorphous silica to be obtained bymeans of sol-gel method (a second method); a method of enhancing thepurity of the ingot to be obtained by melting a raw material (a thirdmethod); and a method consisting of a combination of two or more of theaforementioned first to third methods (a fourth method).

It is possible according to these methods to decrease the content ofmetallic impurities in a quartz glass down to 0.3 ppm or less, or downto 0.1 ppm or less depending on the kinds of metallic impurities. When aquartz glass of such a high purity is employed as a structural materialfor a furnace tube, the metallic contamination (transfer) of a siliconwafer can be minimized, and hence the percent defective of asemiconductor device can be correspondingly minimized.

At present, although it is possible to manufacture a quartz glass havinga metallic impurity content of 0.3 ppm (0.1 ppm), it is still impossibleto manufacture a quartz glass which is completely free from metallicimpurities, i.e. the technique of enhancing the purity of quartz glassis now being nearly deadlocked.

Even if it has become possible to manufacture a quartz glass which iscompletely free from metallic impurities so as to solve the problem ofcontamination with metallic impurities originating from theaforementioned second cause, it is still impossible to solve the problemof contamination with metallic impurities originating from theaforementioned first cause.

Further, in view of a recent trend to further increase the integrationof integrated circuit, a higher cleanness in the process ofmanufacturing a 1G bit DRAM, etc. than at present is now demanded. Inorder to meet such a demand, it is required not only to minimize themetallic impurities in a quartz glass so as to manufacture a high-purityquartz glass, but also to prevent a high-purity quartz glass from beingcontaminated by metallic impurities coming from outside the quartzglass.

As mentioned above, according to the conventional heat treatingapparatus, even if the purity of quartz glass to be employed as astructural material of a furnace tube is enhanced, it has beenimpossible to prevent the furnace tube (quartz glass) from beingcontaminated by metallic impurities coming from various members disposedoutside the quartz glass, thus resulting in the contamination of asilicon wafer.

BRIEF SUMMARY OF THE INVENTION

Therefore, an object of this invention is to provide a quartz glass,which would not become a source for the contamination even if itcontains metallic impurities.

Another object of this invention is to provide a method of manufacturinga quartz glass, which would not become a source for the contaminationeven if it contains metallic impurities.

A further object of this invention is to provide a heat treatingapparatus, which is capable of minimizing the contamination of asemiconductor wafer with metallic impurities.

A still further object of this invention is to provide a heat treatingmethod, which is capable of minimizing the contamination of asemiconductor wafer with metallic impurities.

Namely, according to the present invention, there is provided a quartzglass comprising a region where a concentration of E′ center as measuredby means of an electron spin resonance analysis is 3×10¹⁹ cm⁻³ or more.

According to the present invention, there is further provided a methodof manufacturing a quartz glass which comprises the steps of;

forming an initial quartz glass by melting and quenching a raw materialfor quartz glass; and

implanting therein an ion, which is capable of entering into an SiO₂network of the initial quartz glass and substantially incapable ofexternally diffusing, to increase a concentration of E′ center in atleast part of the initial quartz glass.

There is further provided a method of manufacturing a quartz glass whichcomprises the steps of;

mixing 0.01 to 0.1% by weight of silicon into a raw material for quartzglass;

melting the raw material for quartz glass mixed with the silicon toobtain a melt; and

quenching the melt.

This invention further provides a method of manufacturing a quartz glasswhich comprises the steps of;

melting a raw material for quartz glass to obtain a melt;

quenching the melt thereby to form an initial quartz glass; and

irradiating ultraviolet-rays to the initial quartz glass.

According to the present invention, there is further provided a methodof manufacturing a quartz glass which comprises the steps of;

melting a raw material for quartz glass to obtain a melt;

quenching the melt thereby to form an initial quartz glass; and

giving an abrasion damage to a surface of the initial quartz glass byapplying a sand blast to the surface of initial quartz glass.

According to the present invention, there is further provided a heattreating apparatus comprising;

a furnace tube formed of quartz glass comprising a region where aconcentration of E′ center as measured by means of an electron spinresonance analysis is 3×10¹⁹ cm⁻³ or more; and

heating means mounted around the furnace tube.

According to the present invention, there is further provided a methodof heat-treating a semiconductor wafer, which comprises the steps of;

arranging the semiconductor wafer in a furnace tube formed of quartzglass comprising a region where a concentration of E′ center as measuredby means of an electron spin resonance analysis is 3×10¹⁹ cm⁻³ or more;and

heat-treating the semiconductor wafer while flowing a non-oxidizing gasin such a manner that the non-oxidizing gas contacts directly with anouter wall of the furnace tube.

According to the present invention, there is further provided a methodof heat-treating a semiconductor wafer, which comprises the steps of;

arranging the semiconductor wafer in a furnace tube formed of quartzglass comprising a region where a concentration of E′ center as measuredby means of an electron spin resonance analysis is 3×10¹⁹ cm⁻³ or more;and

heat-treating the semiconductor wafer while irradiating ultraviolet raysonto the furnace tube.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinbefore.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a cross-sectional view of a quartz glass board which has beenemployed in an experiment to demonstrate the principle of thisinvention;

FIG. 2 is a graph-illustrating a relationship between the concentrationof E′ center and an average quantity of trapped Cu;

FIGS. 3A and 3B are cross-sectional views sequentially illustrating thesteps of manufacturing a quartz glass board according to a secondexample of this invention;

FIG. 4 is a graph illustrating a relationship between the concentrationof E′ center and a quantity of implanting silicon ion (Si⁺);

FIG. 5 is a cross-sectional view of a quartz glass board according to athird example of this invention;

FIGS. 6A and 6B are diagrams illustrating the profiles of Cuconcentration when E′ center region is formed only the surface region ofthe glass board and when E′ center region is formed throughout theentire region of the glass board, respectively;

FIGS. 7A and 7B are cross-sectional views sequentially illustrating thesteps of manufacturing a quartz glass board according to a fourthexample of this invention;

FIGS. 8A and 8B are cross-sectional views sequentially illustrating thesteps of manufacturing a quartz glass board according to a fifth exampleof this invention;

FIG. 9 is a cross-sectional view of the heat treating furnace of a heattreating apparatus according to a sixth example of this invention;

FIGS. 10A and 10B are cross-sectional views each illustrating specifican embodiment of the furnace tube of the heat treating furnace shown inFIG. 9;

FIG. 11 is a cross-sectional view of the heat treating furnace of a heattreating apparatus according to a seventh example of this invention; and

FIG. 12 is a cross-sectional view of the heat treating furnace of a heattreating apparatus according to an eighth example of this invention.

DETAILED DESCRIPTION OF THE INVENTION

According to a first embodiment of this invention, there is provided aquartz glass comprising a region where a concentration of E′ center asmeasured by means of an electron spin resonance analysis is 3×10¹⁹ cm⁻³or more.

The term “E′ center” herein means silicon atom which has three balancebonds each having oxygen atom bonded thereto and one balance bondwithout any atoms.

There is not any particular limitation regarding the upper limit of theconcentration of E′ center. However, if the concentration of E′ centerexceeds over 1×10²² cm⁻³, the strength of the quartz glass may belowered. Accordingly, the concentration of E′ center is preferably atmost 1×10²² cm⁻³. More preferably, the concentration of E′ center is7×10¹⁹ cm⁻³ or more.

It has been found as a result of studies made by the present inventorsthat metallic impurities can be effectively trapped by a region (E′center region) where the concentration of E′ center is 3×10¹⁹ cm⁻³ ormore. If a quartz glass is consisted of the one which contains such anE′ center region, the metallic impurities existing in the interior ofthe quartz glass can be prevented from being desorbed outside, and hencethe quartz glass can be prevented from becoming a contamination source.

Additionally, even if metallic impurities coming from outside adhereonto the surface of a quartz glass and diffuse into the interior of thequartz glass, the metallic impurities can be effectively trapped by theE′ center region, so that the quartz glass can be prevented, also inthis case, from becoming a contamination source.

According to a second embodiment of this invention, there is provided amethod of manufacturing the aforementioned quartz glass. As for themethod of manufacturing the aforementioned quartz glass, the followingmethods can be adopted.

(1) A method comprising the steps of forming an initial quartz glass bymelting and quenching a raw material for quartz glass, and implantingtherein an ion, which is capable of entering into an SiO₂ network of theinitial quartz glass and substantially incapable of externallydiffusing, to increase a concentration of E′ center in at least part ofthe initial quartz glass.

Of course, the ions to be implanted into the initial quartz glass shouldpreferably be selected from those which are completely incapable ofdiffusing outside. However, as a matter of fact, such ions are notexisted in a strict sense. Therefore, the expression of “substantiallyincapable of externally diffusing” is employed in this invention.

Specific examples of ions to be implanted into the initial quartz glassare silicon, nitrogen, carbon and aluminum. By the way, by theexpression of “initial quartz glass”, it is meant a state of quartzglass prior to intentionally form a region containing the E′ center.

The dosage of the ion to be implanted into the initial quartz glassshould preferably be 5×10¹⁴ cm⁻² or more, more preferably 1×10¹⁵ ormore, most preferably in the range of from 1×10¹⁵ cm⁻² to 1×10¹⁶ cm⁻².

It is also possible to form the E′ center region throughout the entireregion of the quartz glass, thereby making it possible to moreeffectively trap the metallic impurities existing inside the quartzglass.

(2) A method comprising the steps of mixing 0.01 to 0.1% by weight ofsilicon into a raw material for quartz glass, melting the raw materialfor quartz glass mixed with the silicon to obtain a melt, and finallyquenching the melt.

(3) A method comprising the steps of melting a raw material for quartzglass to obtain a melt, quenching the melt thereby to form an initialquartz glass, and irradiating ultraviolet-rays to the initial quartzglass.

(4) A method comprising the steps of melting a raw material for quartzglass to obtain a melt, quenching the melt thereby to form an initialquartz glass, and giving an abrasion damage to a surface of the initialquartz glass by applying a sand blast to the surface of initial quartzglass.

The concentration of E′ center is measured immediately after the abovemethods are performed.

According to a third embodiment of this invention, there is provided aheat treating apparatus for a semiconductor wafer, which comprises afurnace tube formed of the aforementioned quartz glass. This heattreating apparatus may comprises ultraviolet-irradiating means forirradiating ultraviolet rays to the furnace tube.

According to a fourth embodiment of this invention, there is provided amethod of heat-treating a semiconductor wafer by making use of theaforementioned heat treating apparatus.

According to this heat treating method, the heat treatment is performedwhile flowing a non-oxidizing gas in such a manner that thenon-oxidizing gas contacts directly with an outer wall of said furnacetube comprising the aforementioned quartz glass. The reason for flowinga non-oxidizing gas in this method is to perform a heat treatmentwithout diminishing the E′ center, i.e. in the presence of the E′center.

In this case, the concentration of oxidizing gas that may be included inthe non-oxidizing gas may preferably be 100 ppm or less. If the contentof oxidizing gas is more than 100 ppm, the E′ center may be restored bythe oxygen in the oxidizing gas, thus decreasing the quantity of the E′center. As a result, the quantity of keep the metallic impurities in thequartz glass may be diminished. As for the non-oxidizing gas, nitrogengas, hydrogen gas and an inert gas such as argon gas, etc. may beemployed.

By the way, an ion implantation method or a back side damaging methodhas been conventionally known as a gettering method of a siliconsubstrate. The damage layer to be formed by these methods is amorphous.However, since a silicon substrate is fundamentally formed of singlecrystal, the damage layer may be restored to single crystal under anyheat treatment conditions.

Namely, it is impossible in the case of a single crystal siliconsubstrate to achieve an effect of effectively trapping metallicimpurities. Whereas, in the case of a quartz glass which is inherentlyamorphous, if the heat treatment is performed in a non-oxidizingatmosphere, the amorphous damage layer (a region where the concentrationof E′ center is 3×10¹⁹ cm⁻³ or more) may be remained as it is, thusmaking it possible to ensure an effect of effectively trapping metallicimpurities.

In the case of this invention, the concentration of E′ center (3×10¹⁹cm⁻³ or more) in a quartz glass is very important. This concentration ofE′ center can be easily controlled, since the quartz glass is amorphous.However, since the silicon substrate is formed of single crystal, it isalmost impossible to control the concentration of the E′ center.

This invention will be further explained with reference to the followingexamples and to the drawings.

EXAMPLE 1

First of all, a quartz glass board according to a first embodiment ofthis invention will be explained.

FIG. 1 shows a quartz glass board which has been employed in anexperiment to illustrate the fundamental principle of this invention.Referring to FIG. 1, the reference numeral 1 denotes a quartz glassboard, on the surface of which a region 2 containing the E′ center(hereinafter referred to as an E′ center region 2) is formed. This E′center region 2 has been formed by implanting silicon ions into thesurface of the quartz glass board 1.

The quartz glass board 1 having the E′ center region 2 formed therein isthen introduced into a vacuum vessel together with a silicon wafer whosesurface has been deposited with a Cu layer. Then, the quartz glass board1 and the silicon wafer are subjected to a heat treatment in a vacuumatmosphere at a temperature of 800° C. for 30 minutes.

As a result of this heat treatment, Cu is scattered from the siliconwafer and then adsorbed on the surface of the quartz glass board 1. TheCu thus adsorbed on the surface of the quartz glass board 1 was thendiffused into the interior of the quartz glass board 1, but was trappedas it was diffused into a depth of 150 nm to 400 nm from the surface ofthe quartz glass board 1.

Then, another experiment was performed in the same manner as mentionedabove except that the concentration of E′ center of the E′ center region2 was changed. As a result, the quantity of Cu that has been trapped inthe quartz glass board 1 was also found as being changed. Therefore, arelationship between the concentration of E′ center and an averageconcentration of Cu (hereinafter referred to as an average quantity ofCu capture) at a region locating at a depth of from 100 to 400 nm fromthe surface of the quartz glass board 1 was investigated. In this case,the concentration of E′ center of the E′ center region 2 was measured bymeans of an electron spin resonance analysis.

FIG. 2 shows the results of this experiment. As seen from FIG. 2, it hasbeen found that when the concentration of E′ center in the quartz glassboard 1 was 3×10¹⁹ cm⁻³ or more, the average quantity of Cu capture wassharply increased, and when the concentration of E′ center was 7×10¹⁹cm⁻³ or more, the average quantity of Cu capture was further increased.

Therefore, it will be understood that by employing a quartz glass board1 having an E′ center concentration of 3×10¹⁹ cm⁻³ or more, morepreferably 7×10¹⁹ cm⁻³ or more, the contamination of a silicon wafer dueto Cu contained in the quartz glass board 1 can be effectivelyprevented. In this case, the Cu contained in the quartz glass board 1may be the one which has been originally existed in the quartz glassboard or may be the one which has come from outside and diffused intothe interior of the quartz glass board 1.

In this example, a board of quartz glass was exemplified. However, thisinvention is also applicable to other kinds of shape such as a tubularquartz glass.

EXAMPLE 2

FIGS. 3A and 3B are cross-sectional views sequentially illustrating thesteps of manufacturing a quartz glass board according to a secondexample of this invention.

First of all, as shown in FIG. 3A, quartz was allowed to melt accordinga well known method, and then quenched to form an initial quartz glass11. In this example, a board of initial quartz glass (an initial quartzglass board) was manufactured. However, this invention is alsoapplicable to other kinds of shape such as a tubular quartz glass.

Subsequently, as shown in FIG. 3B, silicon ions were implanted into thesurface of the initial quartz glass board 11 whereby to form an E′center region 13 in the surface region of the initial quartz glass board11, thus manufacturing a quartz glass board.

Then, another quartz glass board was prepared in the same manner asmentioned above except that the quantity of silicon ions implanted waschanged. As a result, the concentration of E′ center was also found asbeing changed. Therefore, a relationship between the concentration of E′center and the quantity of silicon ions implanted was investigated.

FIG. 4 shows the results of this experiment. As seen from FIG. 4, it hasbeen found that when the dosage of silicon ion implanted was 5×10¹⁴ cm⁻²or more, an E′ center region having a concentration of 3×10¹⁹ cm⁻³ ormore in E′ center can be formed, and that when the dosage of silicon ionimplanted was 5×10¹⁶ cm⁻² or more, an E′ center region having aconcentration of 7×10¹⁹ cm⁻³ or more in E′ center can be formed.

Therefore, it will be understood that by ion-implanting silicon ion(Si⁺) at a dosage of 5×10¹⁴ cm⁻² or more, more preferably at a dosage of5×10¹⁶ cm⁻² or more, it is possible to obtain a quartz glass board whichis capable of effectively preventing the contamination of a siliconwafer due to metallic impurities such as Cu which may exist in theinterior of the quartz glass.

In this example, silicon ion was implanted for the purpose of formingthe E′ center region. However, other kinds of ion such as nitrogen,carbon, aluminum, etc. may be substituted for the silicon ion.

The point is to implant an ion which is capable of increasing theconcentration of the E′ center in the interior of the initial quartzglass, and of entering into the SiO₂ network of the initial quartzglass, and is incapable of easily diffusing outside.

EXAMPLE 3

Next, a method of manufacturing a quartz glass board according to athird example of this invention will be explained.

First of all, natural quartz crystal powder (raw material for quartzglass) having a particle diameter of 100 gm and a high purity siliconpowder having a particle diameter of 100 gm were sufficiently mixedtogether to obtain a mixture. The content of the high purity siliconpowder was set to 0.1% by weight. Then, this mixture was melted andquenched to form a quartz glass.

By the way, the high purity silicon powder may be mixed, in place of thenatural quartz crystal powder, with an ordinary raw material for quartzglass such as a synthetic silica powder to be obtained by means of asol-gel method or silicon tetrachloride powder.

According to this method, it is possible, as shown in FIG. 5, to form anE′ center region 22 having a concentration of 3×10¹⁹ cm⁻³ or more of E′center throughout the quartz glass board 21. When the E′ center region22 is formed throughout the quartz glass board 21, the contamination bymetallic impurities such as Cu existing in the interior of the quartzglass board 21 can be more effectively prevented.

By the way, although the concentration of the high purity silicon powderwas set to 0.1% by weight in this example, it is possible to easily formthe E′ center region 22 having a concentration of 3×10¹⁹ cm⁻³ or more ofE′ center by setting the concentration of the high purity silicon powderto not less than 0.01% by weight.

However, if the concentration of the high purity silicon powder exceedsover 0.1% by weight, it may become difficult to keep a stabilized stateof the quartz glass, thus making it difficult to utilize the quartzglass as a structural material of a heat treating apparatus. Therefore,the concentration of the high purity silicon powder should preferably bein the range of from 0.01% by weight to 0.1% by weight.

FIG. 6A shows a profile of Cu concentration when the E′ center regionwas formed only the surface region of the quartz glass, whereas FIG. 6Bshows a profile of Cu concentration when the E′ center region was formedthroughout the quartz glass.

EXAMPLE 4

FIGS. 7A and 7B are cross-sectional views sequentially illustrating thesteps of manufacturing a quartz glass board according to a fourthexample of this invention.

First of all, as shown in FIG. 7A, quartz was allowed to melt accordinga well known method, and then quenched to form an initial quartz glass31. In this example, a board of initial quartz glass (an initial quartzglass board) was manufactured. However, this invention is alsoapplicable to other kinds of shape such as a tubular quartz glass.

Subsequently, as shown in FIG. 7B, the surface of the initial quartzglass board 31 was irradiated with ultraviolet ray 32 of 245 nm inwavelength for one minute, whereby to form an E′ center region 33 havinga concentration of 3×10¹⁹ cm⁻³ or more in E′ center in the surfaceregion of the initial quartz glass board 31, thus manufacturing a quartzglass board. As a result, almost the same effect as that of Example 2was obtained in this example.

In this example, ultraviolet ray was irradiated for the purpose offorming the E′ center region 33. However, it is also possible to employ,in place of ultraviolet ray, an electromagnetic wave such as X-ray,γ-ray, laser beam, etc. or a particle beam such as electron beam.

EXAMPLE 5

FIGS. 8A and 8B are cross-sectional views sequentially illustrating thesteps of manufacturing a quartz glass board according to a fifth exampleof this invention.

First of all, as shown in FIG. 8A, quartz was allowed to melt accordinga well known method, and then quenched to form an initial quartz glass41. In this example, a board of initial quartz glass (an initial quartzglass board) was manufactured. However, this invention is alsoapplicable to other kinds of shape such as a tubular quartz glass.

Subsequently, as shown in FIG. 8B, the surface of the initial quartzglass board 41 was subjected to a blast of silicon carbide powder 42having a particle diameter of 80 μm for 10 seconds at a pressure of 3kg/cm2, whereby to form an E′ center region 43 having a concentration of3×10¹⁹ cm⁻³ or more in E′ center in the surface region of the initialquartz glass board 41, thus manufacturing a quartz glass board. As aresult, almost the same effect as that of Example 2 was obtained in thisexample.

EXAMPLE 6

FIG. 9 is a cross-sectional view illustrating a main portion (heattreating furnace) of heat treating apparatus according to a sixthexample of this invention.

A first feature of this example resides in the employment of a tubularquartz glass comprising an E′ center region having a concentration of3×10¹⁹ cm⁻³ or more in E′ center as a furnace tube 51. It is mostpreferable to form the E′ center region throughout the furnace tube 51.However, even if the E′ center region is existed at least partially inthe furnace tube 51, it is effective in improving the contamination withmetallic impurities.

The furnace tube 51 may be formed of a 2-ply structure consisting of aquartz glass according to this invention and a quartz glass according tothe prior art.

Namely, as shown in FIG. 10A, the inner wall of the furnace tube 51 maybe constituted by a tubular quartz glass 51 a comprising an E′ centerregion having a concentration of 3×10¹⁹ cm⁻³ or more in E′ center, andthe outer wall of the furnace tube 51 may be constituted by a tubularquartz glass (ordinary quartz glass) 51 b having a concentration of lessthan 3×10¹⁹ cm⁻³ in E′ center.

Alternatively, as shown in FIG. 10B, the inner wall of the furnace tube51 may be constituted by a tubular quartz glass 51 b, and the outer wallof the furnace tube 51 may be constituted by a tubular quartz glass 51a.

A second feature of the heat treating apparatus shown in FIG. 9 residesin the provision of a gas flow passage for flowing a non-oxidizing gas53 containing 100 ppm or less of an oxidizing gas, which is formedbetween the furnace tube 51 and a heat-equalizing tube 52 disposedexternally to the furnace tube 51 so as to render the outer wall of thefurnace tube 51 to be directly contacted with the non-oxidizing gas 53.The non-oxidizing gas 53 is allowed to enter through a gas inlet tube 54into the gas flow passage and discharged from a gas outlet tube 55.

The reason for limiting the content of the oxidizing gas in thenon-oxidizing gas 53 is that the concentration of the E′ center in thequartz glass constituting the furnace tube 51 is diminished by a heattreatment in an oxidizing gas atmosphere. Specific examples of thenon-oxidizing gas 53 are nitrogen gas, hydrogen gas and an inert gassuch as argon gas, etc.

The reference numeral 56 in FIG. 9 denotes a susceptor. Other structuralmembers not shown herein may be the same as those of the conventionalapparatus.

The heat treating method by making use of this heat treating apparatuscan be performed as follows. Namely, a silicon wafer (not shown) placedin the furnace tube 51 is heat-treated by a heating means (not shown)while allowing an non-oxidizing gas to flow through a passage betweenthe furnace tube 51 and the heat-equalizing tube 52 disposed externallyto the furnace tube 51 at a flow rate of 16 liter per minute forexample.

During this heat treatment, the metallic impurities originally existedin the furnace tube 51 are trapped by the E′ center region in the quartzglass constituting the furnace tube 51, and also the metallic impuritieswhich have been desorbed from the constituent members (such as theheat-equalizing tube 52) other than the furnace tube 51 and adsorbed(adhered) on the outer wall of the furnace tube 51 are trapped by the E′center region in the quartz glass as the metallic impurities diffuseinto the interior of the furnace tube 51.

Therefore, the metallic impurities can be scarcely desorbed from theinner wall of the furnace tube 51, whereby solving the problem ofcontamination of silicon wafer due to metallic impurities. As a result,the percent defective of semiconductor element can be sufficientlyminimized.

Further, since a non-oxidizing gas flow is employed in this example, thereduction of the E′ center region in the quartz glass constituting thefurnace tube 51 during the heat treatment can be effectively prevented.As a result, the effect of preventing the desorption of metallicimpurities can be sufficiently assured during the heat treatment, andhence any possibility that a silicon wafer is contaminated by metallicimpurities can be extremely minimized.

Moreover, the metallic impurities that have been desorbed from the linertube 52 can be discharged, together with the non-oxidizing gas, from thegas outlet tube 55 to the outside of the apparatus. This alsocontributes to the minimization of the contamination by metallicimpurities.

EXAMPLE 7

FIG. 11 is a cross-sectional view illustrating a main portion (heattreating furnace) of heat treating apparatus according to a seventhexample of this invention. The same portions as those of the heattreating apparatus of FIG. 9 are indicated by the same referencenumerals in FIG. 11 whereby omitting the detailed illustrations thereof(the same in the other examples).

The features of the heat treating apparatus of this example which differfrom the sixth example are that a 2-ply tube 57 was employed as afurnace tube and that a gas flow passage for flowing a non-oxidizing gas53 containing 100 ppm or less of an oxidizing gas is formed between theinner tube and the outer tube of the 2-ply tube 57. The 2-ply tube 57 isconstituted, as in the case of the furnace tube 51, by a quartz glasscomprising an E′ center region having a concentration of 3×10¹⁹ cm⁻³ ormore in E′ center.

The heat treating method by making use of this heat treating apparatuscan be performed as follows. Namely, a silicon wafer (not shown) placedin the 2-ply tube 57 is heat-treated by a heating means (not shown)while allowing an non-oxidizing gas to flow through the passage betweenthe inner tube and the outer tube of the 2-ply tube 57 at a flow rate of18 liter per minute for example.

During this heat treatment, the metallic impurities originally existedin the outer tube of the 2-ply tube 57 are trapped by the E′ centerregion in the outer tube of the 2-ply tube 57, so that the metallicimpurities can be substantially prevented from diffusing into the innertube of the 2-ply tube 57 from the outer tube of the 2-ply tube 57.

Likewise, the metallic impurities which have been desorbed from theconstituent members (such as the liner tube 52) other than the 2-plytube 57 and adsorbed (adhered) on the surface of outer tube of the 2-plytube 57 are trapped by the E′ center region in the outer tube of the2-ply tube 57. Accordingly, in this case also, the metallic impuritiescan be substantially prevented from diffusing into the inner tube of the2-ply tube 57 from the outer tube of the 2-ply tube 57.

On the other hand, the metallic impurities which have been desorbed fromthe surface of the outer tube of the 2-ply tube 57 and adsorbed(adhered) on the outer wall of the inner tube of the 2-ply tube 57 aretrapped by the E′0 center region in the inner tube of the 2-ply tube 57as the metallic impurities diffuse toward the inner wall of the innertube. Accordingly, the metallic impurities can be substantiallyprevented from being desorbed from the inner tube of the 2-ply tube 57.

Therefore, it is now possible according to this example to solve theproblem of the contamination of silicon wafer due to metallic impuritiesin the 2-ply tube 57. Additionally, almost the same effects as obtainedin the seventh example can be obtained. This example can be variouslymodified as in the cases of the aforementioned examples.

EXAMPLE 8

FIG. 12 is a cross-sectional view illustrating a main portion (heattreating furnace) of heat treating apparatus according to an eighthexample of this invention.

The main point of the heat treating apparatus of this example whichdiffers from the sixth example is that, instead of flowing anon-oxidizing gas, an irradiation of ultraviolet ray is employed.

Namely, an ultraviolet ray generator 58 is mounted so as to surround thefurnace tube 51. This ultra-violet ray generator 58 is adapted toirradiate ultraviolet ray 59 having a wavelength of 245 nm onto theouter wall of the furnace tube 51.

The heat treating method by making use of this heat treating apparatuscan be performed as follows. Namely, a silicon wafer (not shown) placedin the furnace tube 51 is heat-treated by a heating means (not shown)while irradiating ultraviolet ray 59 having a wavelength of 245 nm ontothe outer wall of the furnace tube 51.

In this case, as a result of the irradiation of the ultraviolet ray 59,the concentration of E′ center in the surface region of the outer wallof the furnace tube 51 is increased or the reduction in concentration ofE′ center can be inhibited.

As a result, the trapping effect of metallic impurities in the surfaceregion of the outer wall of the furnace tube 51 in particular can besufficiently increased or maintained during the heat treatment.Accordingly, the effect of preventing the desorption of metallicimpurities can be sufficiently increased or maintained during the heattreatment, thus prominently minimizing any possibility of contaminatinga silicon wafer with metallic impurities.

In this example, ultraviolet ray was irradiated. However, it is alsopossible to employ, in place of ultraviolet ray, X-ray, γ-ray, electronbeam or laser beam. The diffusion furnace to be employed in this examplemay be a heat treating furnace of hot wall type utilizing a resistanceheating or a heat treating furnace of cold type mainly utilizing aninfrared heating.

This invention should not be construed to be limited to theaforementioned examples. For example, in addition to the aforementionedheat treating apparatus and methods, this invention is also applicableto a low pressure processing apparatus or method, such as an LP-CVDapparatus or method. Specifically, this invention is applicable to anLP-CVD apparatus or method for forming a polysilicon film, an oxidefilm, a nitride film, etc. In these cases, these films can be preventedfrom being contaminated with metallic impurities.

Furthermore, this invention can be applied to an epitaxial growthapparatus or method. Specifically, this invention is applicable to anepitaxial growth apparatus or method for forming an epitaxial siliconfilm. In this case, the epitaxial silicon film can be prevented frombeing contaminated with metallic impurities.

This invention is also applicable, in addition to the film-formingapparatus or method, to a modifying apparatus or method. Specifically,this invention is applicable to a hydrogen annealing apparatus or methodfor modifying a silicon substrate. In this case, the silicon substratecan be prevented from being contaminated with metallic impurities.

This invention is also applicable to a quartz crucible to be employed ina pulling process of a monocrystalline silicon substrate. In this case,the monocrystalline silicon substrate can be prevented from beingcontaminated with metallic impurities.

This invention can be variously modified within a spirit of thisinvention.

As explained above, it is possible according to this invention toeffectively trap metallic impurities by making use of a region having anE′ center at a concentration of 3×10¹⁹ cm⁻³ or more (E′ center region),so that it is now possible to provide a quartz glass which would notbecome a source for the contamination even if the quartz glass containsmetallic impurities.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A heat treating apparatus comprising; a furnacetube formed of quartz glass comprising a region where a concentration ofE′ center as measured by means of an electron spin resonance analysis is3×10¹⁹ cm⁻³ or more; and heating means mounted around said furnace tube.2. The heat treating apparatus according to claim 1, which furthercomprises ultraviolet-irradiating means for irradiating ultraviolet rayonto said furnace tube.